High-efficiency velocity modulation tube employing harmonic prebunching

ABSTRACT

A velocity modulation tube is disclosed. The tube includes beam field interaction circuits arranged along the beam path in the following order: prebuncher, floating buncher resonator, and output circuit. The prebuncher, buncher and output circuits are all tuned for a fundamental mode of resonance in the vicinity of the pass band of the tube. An abnormally long drift space, i.e., greater than 90* and preferably about 120* of reduced plasma angle is provided between interaction gaps of the prebuncher and next succeeding floating buncher, whereby the second harmonic space charge component of the resultant current density modulation of the beam produces favorable bunching of the electrons at the entrance of the buncher cavity to obtain enhanced conversion efficiency for the tube.

United States Patent 72] Inventor Erling L. Lien Los Altos, Calif. [2| Appl. No. 28,792 [22] Filed Apr. 15, 1970 [45] Patented Nov. 23, 1971 73] Assignee Varian Associates Palo Alto, Calif.

[54] HIGH-EFFICIENCY VELOCITY MODULATION TUBE EMPLOYING HARMONIC PREBUNCHING 9 Claims, 9 Drawing Figs. [52] US. Cl 315/539, 315/551. 315/562 [51] Int. Cl l-l0lj 23/20 [50] Field of Search 315/539, 5.43, 5.51, 5.52

[56] References Cited UNITED STATES PATENTS I 2,494,721 1/1950 Robertson..... 315/543 2,579,480 12/1951 Feenberg 3 15/5.43

FINAL BUNCHER Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatman, Jr. Attorney-Stanley 2. Cole ABSTRACT: A velocity modulation tube is disclosed. The tube includes beam field interaction circuits arranged along the beam path in the following order: prebuncher, floating buncher resonator, and output circuit. The prebuncher, buncher and output circuits are all tuned for a fundamental mode of resonance in the vicinity of the pass band of the tube. An abnormally long drift space, i. e., greater than 90 and preferably about 120 of reduced plasma angle is provided between interaction gaps of the prebuncher and next succeeding floating buncher, whereby the second harmonic space charge component of the resultant current density modulation of the beam produces favorable bunching of the electrons at the entrance of the buncher cavity to obtain enhanced conversion efficiency for the tube.

| f 2 PRE BUNCHER BUNCHER BUNCHER OUTPUT I \:L Z\ 5 g LL H7 is I III:11: 1 T ill r- I 4 7 5 l5 l I 6 1DEGREESf+ 69 OUTPUT FINAL BUNCHER BUNCHER BUNCHER sum 1 OF 2 PATENTEDNUV 23 ISYI PRE BUNCHER ERLING L. LIEN AHORNEY FREQUENCY (MHZ) 4 R w m 6 m w @D E@ m G \M B I H -WMM W EM 6 W 0 fl @ovSzwsrE z2w$ z8 W; m 5 1 P l w m 2 W F um/m m. m l m a m N M m 3 9 8 5 Z n W m I I 6 F F mm? A m W mm M m d mw MVIIII m 3 ZS HIGH-EFFICIENCY VELOCITY MODULATION TUBE EMPLOYING HARMONIC PREBUNCHING DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed, in velocity-modulation tubes, to employ a second harmonic prebuncher resonator upstream of a final buncher resonator for velocity modulating the electron stream with a second harmonic voltage of the signal being amplified such that the second harmonic interaction voltage produces favorable bunching of electrons at the final buncher to obtain improved conversion efficiency. More specifically, the second harmonic RF voltage served to move the interbunch electrons toward the electron bunches while also reducing the velocity spread in the resultant electron bunches, whereby the conversion efficiency of the tube was enhanced. Such an electron tube is disclosed and claimed in copending US. application, Ser. No. 767,774, filed Oct. 15, I968, and assigned to the same assignee as the present inventron.

One of the problems with this prior art harmonic prebuncher for improving efficiency was that the use of a second harmonic cavity in the prebuncher system limited the output bandwidth severely. Therefore, it is desirable to provide harmonic prebunching for improved efirciency in such a manner that the bandwidth of the tube is not so severely limited.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved velocity-modulation tube employing harmonic prebunching.

One feature of the present invention is the provision, in a velocity-modulation tube, of a prebuncher disposed upstream of a floating buncher resonator with a drift space between the center of the interaction gap of the prebuncher and the center of the interaction gap of the succeeding floating bunher being greater than 90 of reduced plasma angle such that the second harmonic component of the resultant space-charge density modulation of the beam causes a further bunching of the beam by moving interbunch electrons into the electron bunches, whereby the conversion efficiency of the tube is enhanced.

Another feature of the present invention is the same as the preceding feature wherein the floating buncher resonator is tuned for a fundamental mode of resonance at a frequency higher than the upper frequency edge of the passband of the tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein at least one additional prebuncher is provided upstream of the first-mentioned prebuncher and wherein the first-mentioned prebuncher is a floating resonator to be excited by the modulated beam for velocity modulating the beam passable therethrough.

Another feature of the present invention is the same as any one or more of the preceding features wherein the first mentioned prebuncher is tuned for a fundamental mode of resonance at a frequency lower than the center frequency of the pass band the tube.

Another feature of the present invention is the same as any one or more of the preceding features wherein the drift space between the prebuncher and the next succeeding floating buncher resonator is approximately 120 of reduced plasma angle.

Other features and advantages of the present invention will become apparent upon perusal of the following specification taken in connection with the accompanying drawings wherein:

; BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram of a multicavity klystron amplifier incorporating features of the present invention,

FIG. 2 is a plot of bunched beam current as a function of distance, depicting an electron bunch,

FIG. 3 is a plot of the fundamental and second harmonic space-charge waves making up the current density modulation of the beam of FIG. 2,

FIG. 4 is a plot of electron phase angles in radians versus mornalized distance in radians along the beam path in the region between the prebuncher cavity of FIG. I and the output p FIG. 5 is a plot of a normalized RF beam current for the fundamental and second harmonic components of the beam current as a function of normalized distance in radians along the axis of the beam for the modulated beam of FIG. 4,

FIG. 6 is a plot of RF conversion efficiency versus normalized load conductance for the output circuit of the tube of FIG. 1,

FIG. 7 is a schematic line diagram depicting an alternative klyston amplifier incorporating features of the present invention,

FIG. 8 is a plot of gain in db. versus frequency depicting the output response for the tube of FIG. 7 and depicting the tuning and Q characteristics for the cavities of FIG. 7, and

FIG. 9 is a schematic line diagram of an alternative velocitymodulation tube of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a klystron amplifier tube 1 incorporating features of the present invention. The tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons 3 over an elongated beam path to a collector structure 4 disposed at the terminal end of the beam 3. A reentrant prebuncher cavity 5 is disposed at the upstream end of the beam 3 and is excited with radio frequency energy to be amplified by an input coupling loop 6. The prebuncher cavity 5 includes an interaction gap 7 defined between the free ends of the reentrant drift tube portions of the reentrant cavity 5, such that the electric fields of the excited resonator interact with the electron beam 3 for velocity modulating the beam.

A conductive drift tube member 8 surrounds the electron beam 3 in the region downstream of the first prebuncher cavity 5 to provide a radio frequency field-free region within which the electrons may drift with velocities imparted to the electrons by the velocity modulation imposed by the prebuncher cavity 5. In addition, space-charge forces, present in the bunched beam further influence the bunching of the electrons as they drift through the drift tube 8 to a succeeding downstream buncher cavity 9.

Buncher cavity 9 is a floating resonator which is defined as a resonator which does not have a source of energy external to the klystron and which is not coupled to a load utilizing the output of the resonator; however, this does not preclude a circuit element coupled to the floating resonator solely for affecting some electrical characteristic of the floating resonator, such as its 0 or frequency. Buncher cavity 9 is of the reentrant-type having an interaction gap 11 defined between the free end portions of the reentrant drift tube members for interacting the electric fields of the resonator 9 with the electron beam passable therethrough. The function of the first buncher cavity 9 is to be excited into resonance by the current density modulated (bunched) electron steam 3 as it passes through the resonator, such excitation of the cavity producing an electric field in the interaction gap 11 for interacting back on the electrons to velocity modulate same, thereby further enchancing the bunching of the electron beam downstream of the first buncher 9.

A second buncher cavity 12, substantially identical to the first buncher cavity 9, is disposed downstream of the first buncher 9 for interacting with the electron stream to further enhance the bunching of the beam. An output reentrant cavity resonator 13 is disposed downstream of the preceding buncher cavity 12. The output cavity 13 is of the reentranttype having an interaction gap 14 defined between the free end portions of the reentrant drift tube members for interaction with the electron stream. More particularly, the current density modulated electron stream excites resonance of the output resonator 13 and output energy is extracted from the output resonator 13 via an output coupling means 15, such as a coupling loop. The energy extracted from the output resonator 13 is then fed to a suitable utilization device, such as an antenna, not shown. Drift tubes 16 and 17 are provided between cavities 9 and 12 and 12 and 13, respectively, for providing field-free drift regions therebetween.

Second hannonic prebunching is obtained in the tube of FIG. 1 by dimensioning the length of the drift tube 8 such that the normalized drift length between the center of interaction gap 7 and the center of the next succeeding interaction gap 11 of the buncher 9 is greater than a quarter of a reduced plasma wavelength. The normalized drift length is defined as:

wq/v! Where iuq is the reduced plasma frequency in radian per second, v is the DC beam velocity, and l is the physical length of the drift space between the interaction gap 7 and 11, and a reduced plasma wavelength is defined as:

A =21r v/wq The second harmonic prebunching, in the case of the abnormally long drift tube 8, is produced by the second harmonic space-charge field present in the modulated beam. This second harmonic of the space-charge field is used instead of the second harmonic circuit of the prior art as described in the aforecited application 767,774 for enhancing the bunching of the beam.

Referring now to FIGS. 2 and 3, the second harmonic space-charge prebunching forces will be described in greater detail. More specifically, the velocity modulation imparted to the electron stream 3 in the prebuncher cavity produces a current density modulation which in a relatively short length of the succeeding drift space causes the electrons to bunch as shown in FIG. 2. This bunch of space charge can be considered to be composed of a fundamental space-charge wave component as depicted by curve 18, and a second harmonic space-charge wave component, as depicted by curve 19. The fundamental and second harmonic space-charge forces, produced by such fundamental and second harmonic waves 18 and 19, respectively, and shown by the arrow, are acting in the same direction in the vicinity of the center of the electron bunch, but are directed in the opposite direction in the interbunch region.

The electrons in the interbunch region continue, therefore, to drift toward the bunch even after the center portion of the bunch is starting to spread because of the space charge forces. Thus, by permitting the electron bunches to drift for an abnormally long drift space, i.e., greater than 90 and less than I50 and preferably 120 of reduced plasma angle, the electrons in the interbunch region are moved into the bunches without too serious a degradation of the bunching within the electron bunches. The upstream buncher cavity 9 serves to rebunch the bunches of electrons before passage'of the bunched beam into the second buncher cavity 12 and thence into the interaction gap I4 of the output resonator l3.

FIG. I shows a typical arrangement of the cavities 5-13 for the large signal portion of the RF circuit of a high-efficiency klystron amplifier. More specifically, it is to be understood that additional fundamental mode prebuncher cavities, similar to cavity 5, would normally be disposed upstream of a final buncher cavity 5 to provide additional gain. Such additional prebuncher cavities would be tuned to different frequencies within the pass band of the tube and would also improve the frequency response, i.e., broaden the pass band of the tube, as more fully described below with regard to FIGS. 7 and 8. In a typical example, the normalized drift length between the center of the gap of the downstream prebuncher cavity 5 and the upstream final buncher cavity 9 is 120 of reduced plasma angle, whereas the corresponding drift lengths between the center of interaction gaps 11 and 11 and I1 and 14 of the buncher cavities are 60 and 25 respectively, of a reduced plasma angle.

Referring now to FIG. 4, there is shown the electron arrival phase angle in radians of 32 reference electrons used within an RF period versus the normalized distance in radians along the beam path through the various interaction gaps for the tube of FIG. 1. Electron phase angle is taken relatively to a reference electron, moving with the DC velocity of the beam, and it is seen that a very favorable bunching of the electron beam is obtained at the output gap.

Referring now to FIG. 5, there is shown the normalized amplitudes of the fundamental component of the RF beam current (l /I and the second harmonic component of the RF beam current (lg/Io) as a function of the normalized distance along the'beam path. It is seen that the normalized fundamental component of beam current reaches a maximum of approximately 1.8 at the output gap and this leads to relatively high calculated conversion efiiciency as shown by curve 20 in FIG. 6. In FIG. 6 it is seen that the calculated conversion efficiency approaches percent at a nonnalized load conductance for the output resonator 13 of 1.0. The load conductance of the output resonator G is normalized to the DC beam conductance G This conversion efiiciency, plotted in FIG. 6, is for a beam having a microperveance of l and the corresponding conversion efficiency for a beam of micropervenace 0.5 is approximately 5 percentage points higher, namely percent.

Referring now to FIGS. 7 and 8, there is shown, in FIG. 7, the arrangement of resonators for a wide band klystron amplifier incorporating the large-signal portion of the RF circuit as shown in FIG. I. In the case of the tube of FIG. 7, two additional prebuncher cavities namely 21 and 22 are disposed upstream of the floating downstream prebuncher cavity 5. Resonator 21 is a floating resonator and resonator 22 is the input resonator supplied with microwave energy, to be amplified, by the input-coupling loop 6. The cavities, numbered in order of sequence from the upstream end to the downstream end of the beam, namely numbers 1 through 6, are tuned to frequencies as indicated by the corresponding arrows numbered 1 through 6 in the plot of FIG. 8 for a tube operating at a center frequency of approximately 12,200 Mhz. The drift lengths between the centers of the interaction gaps of successive resonators is depicted in FIG. 7 as 60, 60, l2090, and 30 respectively as indicated in the diagram. The Q's for the respective cavities are labeled in the plot of FIG. 8 and it is seen that the two final buncher cavities 9 and 12, corresponding to the fourth and fifth resonators, have a relatively high loaded Q, as of 850, and are tuned to frequencies substantially higher than the upper band edge frequency of the pass band of the tube, as determined by the points on the gain curve 31 where the gain is down by 3 db. from the maximum gain at the center of the pass band of the tube. Also it is seen that the downstream prebuncher cavity 5, which is the third cavity of the sequence, is tuned slightly below the lowfrequency band edge frequency and substantially below the center frequency of the pass band of the tube. The input and output resonators labeled 1 and 6 are tuned within the pass band of the tube. A listing of typical design parameters for a tube of the type depicted in FIG. 7 is shown below in table 1.

TABLE I SUMMARY OF DESIGN PARAMETERS Normalized length of the interaction gaps:

Buneher cavities l.0 radian! Output cavity 0.7 radians Length of the interaction gaps:

Buncher cavities 0.033 inches Output cavity 0.023 inches Normalized tunnel radius, a 0.7 rudinns Tunnel diameter, 2a 0.047 inches Beam filling factor, b/n 0.66 Normalized drift lengths, 3

5. M Drift lengths I 0.508 inches 1,, 0.508 inches I 0.761 inches I 0.254 inches Total Q-factor of the cavities:

Om... 130 Cavity tuning:

f, 12.1 s! MHZ 1, 12.222 MHZ f, l2.l7l MHz f, 12.266 MHz 1, l2.283 MHz 12.200 MHz Magnetic focusing field (2.5XBrillouin field) 4000 Gauss Referring now to FIG. 9 there is shown an alternative klystron embodiment of the present invention. This embodiment is substantially the same as that of FIG. 1 with the exception that only a single final buncher cavity 9 is employed between the prebuncher cavity 5 and the output cavity 13. The drift space between the center of the interaction gap 7 of the input resonator 5, which is also the prebuncher resonator, and the interaction gap of the final buncher cavity 9 is greater than 90 of reduced plasma angle and the drift length between the interaction gap 11 in buncher cavity 9 and the interaction gap 14 in the output resonator 13 is 35 of reduced plasma angle.

In all of the embodiments of FIGS. 1, 7, and 9 the resonators are all tuned for a fundamental mode of resonance in the vicinity of the pass band of the tube. One advantage of this invention is that it permits hannonic prebunching to be achieved with cavities tuned for fundamental resonance near the pass band of the tube, as opposed to prior art tubes which employed second harmonic resonators which had to be tuned for a fundamental mode of resonance at the second harmonic of the pass band. At frequencies above S-band an inordinate proportion of the volume of a second harmonic cavity is occupied by the beam.

It is not a requirement that the resonator circuits employed in the tubes of the present invention be r'eentrant cavity resonators. It is contemplated that other types of resonant circuits may be employed, such as distributed field helix resonators (either the single helix or cross-wound helix-type may be employed.) Moreover, the output resonator circuit 13 may comprise, for example, a slow wave circuit or an extended interaction circuit formed by a plurality of coupled cavity resonators. in other words, the present invention is applicable not only to klystron amplifiers but to velocity modulation microwave tubes in general which employ tuned resonant structures disposed along the beam path for interaction with the beam.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is: '1. In a velocity modulation tube having a certain operating pass band of frequencies, means for pro'ecting a beam of electrons over an elongated beam path, pre uncher resonator circuit means disposed on the beam path and having an interaction gap therein for electromagnetic interaction with the beam to velocity modulate the beam passable through said interaction gap, a fundamental frequency floating buncher resonant circuit means disposed on the beam path downstream of and next succeeding said prebuncher means and having an interaction gap therein for excitation by the velocity modulated beam and for further electromagnetic interaction with the beam to velocity modulate the beam passable through said interaction gap thereof, output circuit means disposed on the beam path downstream of said buncher means for excitation by the modulated beam and for extracting output wave energy from the beam, and drift tube means interposed between said prebuncher means and said next succeeding buncher means for shielding the beam from the RF fields of wave energy external of the beam to provide a substantially uninterrupted RF field free drift region within the beam between the centers of said interaction gaps of said prebuncher and said buncher means of a normalized drift length greater than of reduced plasma angle, whereby the RF conversion efficiency of the tube is increased.

2. The apparatus of claim 1 wherein said buncher means is tuned for a fundamental mode of resonance at a frequency higher than the upper band edge frequency of the pass band of the tube.

3. The apparatus of claim 1 including a second floating buncher resonant circuit means disposed on the beam path between said first floating buncher means and said output circuit means for excitation by the modulation on the beam and for velocity modulating the beam passable therethrough, and wherein said first and second buncher means are tuned for a fundamental mode of resonance at a frequency higher than the upper band edge frequency of the pass band of the tube.

4. The apparatus of claim 1 including at least one additional prebuncher resonator means disposed on the beam path upstream of said first-mentioned prebuncher means for velocity modulating the beam with signal wave energy within the pass band of the tube, and wherein said first mentioned prebuncher is a floating resonator to be excited by the modulated beam for velocity modulating the beam passable therethrough.

5. The apparatus of claim 4 wherein said first mentioned prebuncher means is tuned for a fundamental mode of resonance at a frequency lower than the center frequency of the pass band of the tube.

6. The apparatus of claim 1 including additional drift tube means interposed along the beam path between the remaining resonators providing RF field-free drift spaces for the beam therebetween, and wherein each of said additional drift tube means has a normalized drift length less than 90 of reduced plasma angle.

7. The apparatus of claim 1 wherein said normalized drift length is also less than 150 of reduced plasma angle.

8. The apparatus of claim 1 wherein said normalized drift length is approximately of reduced plasma angle.

9. The apparatus of claim 1 wherein said buncher, prebuncher, and output circuit means are all reentrantcavity resonators. 

1. In a velocity modulation tube having a certain operating pass band of frequencies, means for projecting a beam of electrons over an elongated beam path, prebuncher resonator circuit means disposed on the beam path and having an interaction gap therein for electromagnetic interaction with the beam to velocity modulate the beam passable through said interaction gap, a fundamental frequency floating buncher resonant circuit means disposed on the beam path downstream of and next succeeding said prebuncher means and having an interaction gap therein for excitation by the velocity modulated beam and for further electromagnetic interaction with the beam to velocity modulate the beam passable through said interaction gap thereof, output circuit means disposed on the beam path downstream of said buncher means for excitation by the modulated beam and for extracting output wave energy from the beam, and drift tube means interposed between said prebuncher means and said next succeeding buncher means for shielding the beam from the RF fields of wave energy external of the beam to provide a substantially uninterrupted RF field free drift region within the beam between the centers of said interaction gaps of said prebuncher and said buncher means of a normalized drift length greater than 90* of reduced plasma angle, whereby the RF conversion efficiency of the tube is increased.
 2. The apparatus of claim 1 wherein said buncher means is tuned for a fundamental mode of resonance at a frequency higher than the upper band edge frequency of the pass band of the tube.
 3. The apparatus of claim 1 including a second floating buncher resonant circuit means disposed on the beam path between said first floating buncher means and said output circuit means for excitation by the modulation on the beam and for velocity modulating the beam passable therethrough, and wherein said first and second buncher means are tuned for a fundamental mode of resonance at a frequency higher than the upper band edge frequency of the pass band of the tube.
 4. The apparatus of claim 1 including at least one additional prebuncher resonator means disposed on the beam path upstream of said first-mentioned prebuncher means for velocity modulating the beam with signal wave energy within the pass band of the tube, and wherein said first mentioned prebuncher is a floating resonator to be excited by the modulated beam for velocity modulating the beam passable therethrough.
 5. The apparatus of claim 4 wheRein said first mentioned prebuncher means is tuned for a fundamental mode of resonance at a frequency lower than the center frequency of the pass band of the tube.
 6. The apparatus of claim 1 including additional drift tube means interposed along the beam path between the remaining resonators providing RF field-free drift spaces for the beam therebetween, and wherein each of said additional drift tube means has a normalized drift length less than 90* of reduced plasma angle.
 7. The apparatus of claim 1 wherein said normalized drift length is also less than 150* of reduced plasma angle.
 8. The apparatus of claim 1 wherein said normalized drift length is approximately 120* of reduced plasma angle.
 9. The apparatus of claim 1 wherein said buncher, prebuncher, and output circuit means are all reentrant cavity resonators. 